In this specification, including its claims, the term GPS is to be construed broadly, and must be read to include not only the Global Positioning System, but all Global Navigation Satellite Systems (“GNSS”) using CDMA (“Code Division Multiple Access”) technology. Likewise, the terms “jamming” and “interference signals,” which are used interchangeably, are to be construed broadly as well, and include without limitation interference signals emitted by both narrowband and wideband jammers.
Space Time Adaptive Processing (STAP) for antijam has been used to suppress jamming (interfering) signals that may corrupt GPS navigation. The most basic STAP filter design goal is to compute the STAP filter weights vector to do no more than minimize the output power of the filter while avoiding the null weight vector solution. As a convenience, we refer to this herein as Unconstrained Nulling. As the GPS signals are below the receiver system noise floor while jamming signals are commonly above the noise floor, the STAP filter tends to suppress the jammer signal(s) with little attenuation of the GPS signal, unless the GPS signals and jammer signals arrive at the antenna array from the same or similar direction(s). However, in achieving the basic design goal of minimizing the STAP filter output power, the prior art STAP filter typically induces phase (carrier and code) distortion to the GPS signals.
The current industry solution for compensating for STAP-induced code phase and carrier phase bias distortions in a jamming environment involves use of a beamforming-with-nulling architecture to form individual beams toward specific satellites used in the navigation solution; this procedure is large, costly, and complicated. The digital beamforming-with-nulling antijam electronics solution introduces minimum GPS code phase and carrier phase bias errors, as compared to the digital “nulling only” solution. This traditional beamforming approach to suppressing interference and mitigating phase distortion to the desired GPS signals employs beamforming with constraints. The optimal design goal is to minimize the STAP filter array output power while simultaneously fixing the gain and producing an overall linear phase response in the direction of the desired GPS signal. Mathematically, this is posed as a Quadratic Programming Problem (“QPP”) with constraints, usually in the form of equality constraints.
Digital beamforming, however, requires extensive antenna calibrations, a large antenna array, additional digital signal processing complexity for forming individual beams toward specific satellites used for obtaining the navigation solution, changes to the receiver design, and a high speed digital interface between the digital receiver and the digital antenna electronics. This solution ordinarily requires a separate STAP filter for each GPS satellite signal desired to be tracked, resulting in a complex and undesirable signal processing architecture that requires the parallel implementation of multiple STAP filters where the filter order might be high (tens of taps or more), and has multiple independent signal paths (one from each STAP filter) to the GPS receiver.
Since the beamforming solution adds significant complexity to the antenna electronics and requires major modifications to the platform GPS receiver, it is not desirable for SWAP-constrained platforms (SWAP=size, weight, and power), such as ‘small’ unmanned aerial vehicles (UAVs) and rotary wing aircraft. While the digital beamforming antenna electronics and receiver solution with its attended complexity may be acceptable on a large platform, such as a ship or fixed wing aircraft, it does not meet the tight SWAP constraints of a rotary wing aircraft. The rotary wing aircraft application presents additional technology challenges to the digital beamforming anti-jam antenna electronics because of rotor blade jammer reflections and modulation of GPS signals on such platforms, which need to be addressed.
For the above reasons, innovative antenna signal processing solutions are required to address this technology gap for rotary wing aircraft. An effective SWAP optimized antijam solution that works for rotary wing aircraft is needed. Specifically, innovative antenna antijam signal processing solutions are required that work in the rotary wing aircraft environment of rotor blade modulation and jammer reflection, employ a small antenna, for example (and not by way of any limitation) a small Controlled Reception Pattern Antenna array (s-CRPA), mitigate the effect of potential code delay and carrier phase bias introduced by STAP filter antenna nulling solutions, and require minimum to no changes to the platform's legacy GPS receiver.